A method for recording by means of an ink-jet printer, a typical method among various color recording methods, comprises generating ink droplets and depositing them onto various recording materials (such as paper, film and cloth). This method has been rapidly prevailing lately and is expected to grow remarkably in the future because of such features as less noise generation due to no contact of a recording head with a recording material and easiness in downsizing and speedup. Conventionally, as an ink for a fountain pen or a felt pen and an ink for ink-jet recording, a water-based ink dissolving a water-soluble dye in a water-based medium has been used. Therefore, in these water-soluble inks, a water-soluble organic solvent is generally added to prevent ink from clogging at a pen tip or an ink-jet nozzle. These conventional inks are required to provide a recorded image of sufficient density, not to clog at a pen tip or an ink-jet nozzle, to dry quickly on a recording material, to bleed less, to have good storage stability and, in particular, to have high solubility in water and a water-soluble organic solvent to be added to the inks. Moreover, an image formed is required to have image fastness such as water fastness, light fastness, ozone gas fastness and moisture fastness.
Ozone gas fastness means durability against phenomenon that oxidizing ozone gas in the air reacts with a dye on a recording paper to incur discoloration or fading of a printed image. Although oxidizing gas having this kind of action includes NOx and SOx besides ozone gas, ozone gas is said to be a causative substance to promote the phenomenon of discoloration or fading of an ink-jet recorded image, further than these oxidizing gases.
Ozone gas is used for an accelerated test to measure degree of ozone gas fastness. As the phenomenon of discoloration or fading in color caused by these oxidizing gases is specific to an ink-jet image, improving ozone gas fastness has become an important subject. In particular, for an ink-receiving layer mounted at the surface of a paper for exclusive use for ink-jet photographic image, so as to dry the ink faster and decrease bleed on the paper of high image quality, porous materials of inorganic white pigments are often used. The phenomenon of discoloration or fading in color caused by ozone gas occurs noticeably on such recording papers.
Moisture fastness means resistance against phenomenon that a dye colorant on a recording material bleeds while the colored recording material is stored under high-humidity atmosphere. As bleed of a dye colorant significantly impairs the image quality particularly in an image requiring highly precise image quality like a photograph, it is important to make such bleed as little as possible. Therefore, moisture fastness as well as the above mentioned ozone gas fastness is also an important subject to be required in a colorant for ink-jet recording.
To extend application fields of a printing method using ink in the future, an ink composition to be used for ink-jet recording and a colored product thereby are strongly required to exhibit further improved water fastness, light fastness, moisture fastness and ozone gas fastness.
Among inks with various hues prepared from various dyes, a black ink is important for not only in application for printing written information, but also for a color image. However, in the development of a good black colorant having neutral hue in both deep and pale color regions, and high color density and less dependence of hue on a light source, there are technical barriers, and sufficient performance can hardly be found in spite of active research and development. By this reason, various plural colorants are generally mixed to form a black ink. An ink prepared by mixing plural colorants poses such a problem that its hue changes depending on a medium to be used and particularly much changes by decomposed colorant by light or ozone gas.
Black inks prepared by a black dye blended with a yellow to orange dye are proposed in patent literatures such as JP-B-7-122044, JP No. 3178200 and JP Laid-Open No. 255906/1997, however, a product sufficiently satisfying market needs has not yet been provided.
Non-patent literatures such as “Chemistry of Synthesized Dyes” (Kenzo Konishi and Nobuhiko Kuroki, published by Maki Shoten, Mar. 15, 1965, P. 143-145, P. 187-188) and “Theoretical Production, Dye Chemistry” (Yutaka Hosoda, published by Gihoudou Inc., Oct. 1, 1963, P. 190-193) describe a condensate compound (B) of 4,4′-dinitrostilbene-2,2′-disulfonic acid or a salt thereof and aminobenzenes, or reduction product (C) thereof used in the present invention (hereinafter, may be referred to as DNSDS-AB condensate compound for simplicity).
A conjugate azo compound has been proposed in, for example, JP Laid-Open No. 302221/1996, JP Laid-Open No. 12910/1997 and JP No. 2565531, however, a product sufficiently satisfying market needs has not yet been provided.
An object of the present invention is to provide a black ink composition that has stability even in long-term storage and that gives a black recorded image that has a toneless and neutral gray to black color, a printed image of high density and color independence on each medium and is superior in moisture fastness, light fastness and ozone gas fastness.